In the art of chemical pulping of comminuted cellulosic fibrous material, for example wood chips, the cellulose material is typically treated with cooking chemicals under pressure and temperature in one or more cylindrical vessels, known as digesters. This treatment can be performed continuously or in a batch mode. In the continuous mode, chips are essentially continuously fed into one end of a continuous digester, treated, and continuously discharged from the other end. In the batch method, one or more batch digesters are filled with chips and cooking chemical, capped and then treatment commences. Once the treatment is finished the contents of the batch digester are discharged. In either batch or continuous digesters, a slurry of comminuted cellulosic fibrous material and cooking chemical is treated in one or more a cylindrical vessels.
In both continuous and batch digesters, in order to uniformly distribute both temperature and cooking chemical, cooking liquor is typically circulated through the slurry of chips and liquor, typically referred to as "the chip column". This circulation is typically facilitated by some form of screen, located along the internal surface of the cylindrical vessel, and a pump, a heater, and a return conduit. The screen retains the material within the digester as the liquor is removed, augmented with other liquors and/or a portion thereof removed, pressurized, heated, and then returned to the slurry in the vicinity of the screen or elsewhere. The proper operation of the digester and the production of uniform product having the best or optimum properties, for example, strength, are highly dependent upon the efficiency and uniformity of this liquid circulation process.
Typically, digester screen assemblies comprise or consist of right cylindrical screen surfaces of relatively uniform diameter. These screen surfaces may comprise or consist of perforated plate, having slots or holes, or parallel-bar type constructions having parallel apertures between the bars. These bars typically have a substantially vertical orientation, but may have various other orientations including substantially horizontal, or at an oblique angle [for example, at a 45.degree. angle to the vertical].
The uniform removal of liquid using screens has always been a consideration in the design and operation of digesters, both continuous and batch. For example, the radial removal of liquor typically produces radial compression of the chip column in the vicinity of the screen assembly which can cause chips to become lodged in the screen openings. Chips and other debris that pass through the screen can accumulate in the cavities behind the screen. Scale, for example, carbonate scale, which can precipitate from the cooking liquor can also build up on the screen or in the cavities and piping behind the screen.
Various methods have been proposed, some relatively successful, some not, to keep the screen surfaces and the cavities behind them free of chips, debris, and scale. These methods include periodic backflushing of the screens, or--as in U.S. Pat. Nos. 3,589,521, 3,752,319, 3,755,072, and 4,637,878, and Canadian patent 949,460; and Swedish patent 466,706--disclose assorted mechanisms which attempt to somehow agitate the screen surface to attempt to keep it clear.
In earlier digesters, as exemplified in U.S. Pat. Nos. 2,474,863; 2,459,180; and 2,695,232, liquor removed from a digester via a cylindrical screen, or "screen girdle", was collected in an one or more annular cavities behind the screen and then removed from these cavities by means of one or more pipes. U.S. Pat. No. 3,711,367 discloses an improvement upon this system in which the annular cavities behind the screens communicate, via orifices holes, with a common manifold or header adjacent to the screen cavity. These "internal headers" replace the earlier "external header" design and provide a more compact design and a more uniform distribution of liquor removal.
The systems of U.S. Pat. Nos. 4,547,264 and 5,069,752 (the disclosures of which are incorporated by reference herein) provided further improvement to the uniformity of liquor withdrawal and the systems these patents disclose represent the typical existing art. These patents disclose assorted methods of varying the volume and direction of liquid withdrawal for digesters having multiple screen systems. U.S. Pat. No. 4,547,264 and U.S. Pat. No. 5,069,752 disclose methods (referred to as "side-to-side" and "up-down" screen switching and "cross switching" screen switching, respectively) in which an automatic control system periodically initiates and then terminates flow from different sets of screens. Though these systems improve the uniformity of liquor removal and minimize the accumulation of scale and debris, the numerous control valves that are necessary typically require a significant amount of maintenance and thus some form of human access. Typically, platforms with guard rails and access ladders, etc. are erected in the areas below the valves and piping to provide access for maintenance and repair crews. Since the prior art systems include piping and valves distributed 360.degree. around the digester, these expensive platforms must typically be constructed 360.degree. around the digester, at several elevations. These prior art systems also require longer, more expensive pipe headers that run around the vessel.
The present invention overcomes the limitations of the existing art by providing a simplified liquor removal method and system that reduces the amount of piping and the number of control valves required, and thus their associated maintenance, and thus reduces the need for providing for ready human access to the entire digester. For example, the present invention reduces the number of platforms that need be erected to access and maintain the liquor removal valves and thus provides for a less expensive digester installation. The invention can be applied to any screen assembly in the digester, including cooking screens, extraction screens or wash circulation screens. This invention is also not limited to digesters but can be employed in any system, such as pretreatment or impregnation vessels, and washing and bleaching equipment, in which liquid is periodically or continuously removed from a slurry of comminuted cellulosic fibrous material, e.g. through screens.
The invention may comprise an assembly, per se, for use in screening liquid, such as a substantially annular screen assembly for removing liquid from a slurry of comminuted cellulosic fibrous material, the screen assembly having a screen surface, an annular cavity behind said screen surface for collecting liquid passed through the screen, two conduits for removing liquid from the cavity located adjacent to each other, and a barrier between the cavities. The screen surface may comprise a substantially continuous cylindrical screen surface, or have a wide variety of other configurations as is conventional for screen surfaces per se, particularly for screens in chemical pulp digesters.
According to one aspect of the present invention a pulp treatment vessel assembly (typically a digester, such as a continuous or batch digester, but also possibly an impregnation vessel, washing vessel, or other pulp treatment or production vessel) is provided comprising the following components: a substantially vertical vessel having a top and a bottom. A first substantially annular screen assembly disposed within the vessel between the top and the bottom. A first substantially annular header associated with the first screen assembly. First and second withdrawal conduits extending outwardly from the header and in fluid communication therewith. A barrier disposed in the header and disposed between the withdrawal conduits. And, the withdrawal conduits are circumferentially spaced from each other between about 2-30 degrees. Also, the assembly may include a substantially continuous platform having an arcuate extent of less than 180 degrees disposed exteriorly of the vessel and adjacent the withdrawal conduits to allow one (an operator or maintenance worker) on the platform to readily access the conduits and any structures associated therewith.
A conventional automatically operated valve is typically provided in each of the withdrawal conduits, and accessible from the platform, and the withdrawal conduits within any set are typically preferably spaced from each other between about 10-20.degree.. In this case the arcuate extent of the platform is typically between about 80-110.degree., preferably about 90.degree. (or less). Preferably a plurality of (e.g. at least two other) screen assemblies are provided, and each may comprise a pump connected to a common conduit with the first and second withdrawal conduits, automatically controlled valves, disposed in or in association with the first and second withdrawal conduits, a heater connected by a conduit to the pump, and a recirculation conduit extending from the heater to the interior of the digester for recirculating liquid withdrawn through the first and second withdrawal conduits to an interior portion of the digester, or connected to a conduit which simply withdraws liquid and passes it to recovery.
According to another aspect of the present invention an assembly for use in screening liquids is provided comprising the following components: A first substantially annular screen assembly. A first substantially annular header associated with said screen assembly. First and second withdrawal conduits disposed adjacent each other and extending outwardly from said header and in fluid communication therewith. A barrier disposed in said header, and between said withdrawal conduits. And an automatically controlled valve disposed in or in association with each of said withdrawal conduits. The assembly may further comprise the following components: a second annular screen assembly disposed immediately below the first screen assembly; a second substantially annular header associated with the second screen assembly; third and fourth withdrawal conduits extending outwardly from the second header and in fluid communication therewith; an automatically controlled valve disposed in or in association with each of the third and fourth withdrawal conduits; and a barrier disposed on the second header, and between the third and fourth withdrawal conduits. The various withdrawal conduits are preferably spaced as described above, and a platform is associated therewith as described above. The centerlines of the conduit sets are preferably spaced from each other between about 80-110.degree., preferably about 90.degree..
According to another aspect of the present invention a method of treating a liquid slurry of comminuted cellulosic fibrous material in a substantially vertical vessel, such as a continuous digester, and having components such as set forth above, is provided. The method comprises the following steps: (a) Introducing the slurry of comminuted cellulosic fibrous material into the vessel to flow into operative association with the screen assembly between the top and bottom of the vessel. (b) Screening the slurry with the screen assembly to cause some liquid to flow from the slurry into the header. (c) For a first period of time, withdrawing liquid from the first conduit while substantially no liquid is being withdrawn from the second conduit, liquid being withdrawn through the first conduit from adjacent the second conduit moving around the substantially annular header to the first conduit. (d) For a second period of time, withdrawing liquid from the second conduit while substantially no liquid is being withdrawn from the first conduit, liquid being withdrawn through the second conduit from adjacent the first conduit moving around the substantially annular header to the second conduit. (e) Periodically repeating steps (c) and (d). And (f withdrawing treated slurry from the vessel. Steps (c) and (d) may each be practiced for between about 2-6minutes each. In this way the screen of the screen assembly is typically kept free, so that it does not clog significantly.
The method as described above may also include at least one substantially annular screen assembly comprises first and second annular screen assemblies one disposed immediately below the other and each having a header with first and second adjacent conduits extending outwardly therefrom and a barrier in the header between the conduits associated therewith, and wherein steps (c) and (d) are practiced for the conduits associated with both said first and annular screen assemblies. The first conduits of the first and second annular screen assemblies are preferably circumferentially adjacent each other (typically not spaced more than about 20.degree.-40.degree.) and the second conduits are likewise circumferentially adjacent each other; and steps (c) and (d) are practiced so that liquid is withdrawn through the first conduits of both the first and second screen assemblies at the same time, and through both the second conduits (of the first and second screen assemblies) at the same time, so that there is only side-to-side switching, and not up-and-down switching.
The method may also include the conduits from said first screen assembly have a centerline that is spaced from the conduits for the second screen assembly less than about 110.degree. (preferably between about 80-110.degree., e.g. about 90.degree.), and wherein a platform having an arcuate extent of about 110.degree. or less is mounted to the vessel adjacent the conduits, valves, and/or instrumentation of the annular screen assemblies to allow ready access thereto; and comprising the further step of a human operator accessing the platform and from the platform servicing, repairing, or replacing the conduits, valves, and/or instrumentation of the first and second annular screen assembly or structures associated therewith.
There is also preferably the further step of repeating steps (b)-(c) at least once (and typically 2-3 times) prior to step (f). There may also be the further step of heating the liquid removed in the practice of steps (c) and (d), and reintroducing the heated liquid into the digester adjacent where it was removed. As is conventional, some of the liquid flow may be removed, and/or other liquid added, prior to return to the digester, or other vessel.
It is the primary object of the present invention to provide a simplified screen assembly, and method of utilization thereof, for a comminuted cellulosic fibrous material treatment vessel which allows for reduced access and maintenance such that the number or extent of access platforms may be reduced. This and other objects of the invention will become clear from an inspection of the detailed description of the drawings and from the appended claims.